Patent application title: IMAGE DISPLAY APPARATUS

Abstract:

An image display apparatus includes a light source, a color separation
unit configured to separate light from the light source into colored
light beams, liquid crystal display devices each configured to modulate a
corresponding one of the colored light beams in accordance with an image
signal, light-shielding members each configured to block light that
enters an area outside an effective display area of a corresponding one
of the liquid crystal display devices, a color combination unit
configured to combine the colored light beams that have been modulated by
the liquid crystal display devices, and a projection unit configured to
project light produced by combining the colored light beams with the
color combination unit, wherein the liquid crystal display devices are
fixed to a heatsink, and wherein the light-shielding members each include
a base member fixed to the heatsink and a mask member detachably mounted
on the base member.

Claims:

1. An image display apparatus comprising:a light source;a color separation
unit configured to separate light from the light source into a plurality
of colored light beams;a plurality of liquid crystal display devices each
configured to modulate a corresponding one of the colored light beams in
accordance with an image signal;a plurality of light-shielding members
each configured to block light that enters an area outside an effective
display area of a corresponding one of the liquid crystal display
devices;a color combination unit configured to combine the colored light
beams that have been modulated by the liquid crystal display devices;
anda projection unit configured to project light produced by combining
the colored light beams with the color combination unit;wherein the
liquid crystal display devices are fixed to a heatsink; andwherein the
light-shielding members each include a base member fixed to the heatsink
and a mask member detachably mounted on the base member.

2. The image display apparatus according to claim 1, wherein the base
member has an opening larger than a light entry/exit surface of a
corresponding one of the liquid crystal display devices.

3. The image display apparatus according to claim 1, wherein the base
member and the mask member are detachably joined to each other by
engaging a protrusion disposed on one of the base member and the mask
member with a recess disposed in the other one of the base member and the
mask member.

4. The image display apparatus according to claim 1, further comprising an
elastic member configured to surround a space formed by a corresponding
one of the liquid crystal display devices and an optical element that is
disposed at a side of the light source of the corresponding one of the
liquid crystal display devices.

5. The image display apparatus according to claim 4, wherein the elastic
member is mounted on the light-shielding member.

[0004]In liquid crystal projectors, liquid crystal display devices each
corresponding to a light of a particular color are provided. Near the
surface of each of the liquid crystal display devices, a light-shielding
member is disposed so as to block light from entering the area outside
the effective display area of the liquid crystal display device.

[0005]Positions of the liquid crystal display devices each of which
corresponds to light of a particular color are adjusted with very high
accuracy so that pixels of the liquid crystal display devices correspond
thereto. However, when the accuracy of positioning a light-shielding
frame of the light-shielding member is low relative to the effective
display area of the liquid crystal display device, the relative position
of the light-shielding frame may become misaligned for light of each
color. Accordingly, in the periphery of an image projected on a screen,
the light-shielding frame of the light-shielding member is projected to a
different position for light of each color. Such an image is recognized
as an image in which the colors are smudged.

[0006]In an image display apparatus disclosed in Japanese Patent Laid-Open
No. 11-305674, a light-shielding member of a liquid crystal display
device is fixed with a number of screws to a heatsink. Because the
position of the light-shielding member is not adjustable, the accuracy
with which the position of the light-shielding member is set relative to
the effective display area of the liquid crystal display device may not
be sufficient because of an error in manufacturing and assembling the
liquid crystal display device or the light-shielding member.

[0007]If the light-shielding member is fixed with a simple structure,
removing dust that has become stuck to the surface of the liquid crystal
display device becomes difficult after the position of the
light-shielding member has been adjusted and fixed.

[0008]That is, because the surface of the liquid crystal display device is
covered with the light-shielding frame of the light-shielding member up
to the vicinity of the effective display area, when dust become stuck to
the surface of the liquid crystal display device, it is necessary to
remove the dust by detaching the light-shielding member. Moreover, it is
necessary to readjust the position of the light-shielding member and
reassemble the light-shielding member. Therefore, the dust cannot be
easily removed.

SUMMARY OF THE INVENTION

[0009]According to an aspect of the present invention, a light-shielding
member of an image display apparatus is disposed with high accuracy
relative to an effective display area of each of a plurality of liquid
crystal devices. Therefore, color smudge in the peripheral area of a
projection image is prevented. Moreover, dust on the surface of each
liquid display device can be removed easily.

[0010]According to another aspect of the present invention, an image
display apparatus includes a light source, a color separation unit
configured to separate light from the light source into a plurality of
colored light beams, a plurality of liquid crystal display devices each
configured to modulate a corresponding one of the colored light beams in
accordance with an image signal, a plurality of light-shielding members
each configured to block light that enters an area outside an effective
display area of a corresponding one of the liquid crystal display
devices, a color combination unit configured to combine the colored light
beams that have been modulated by the liquid crystal display devices, and
a projection unit configured to project light into which the colored
light beams have been combined by the color combination unit.

[0011]According to another aspect of the present invention, the liquid
crystal display devices are fixed to a heatsink. The light-shielding
members each includes a base member fixed to the heatsink and a mask
member detachably mounted on the base member.

[0012]Further features of the present invention will become apparent from
the following description of exemplary embodiments with reference to the
attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013]FIG. 1 is a plan view of an optical system of an image display
apparatus according to a first embodiment of the present invention.

[0014]FIG. 2 is a side view of the optical system of the image display
apparatus according to the first embodiment of the present invention.

[0015]FIG. 3 is an exploded perspective view of the image display
apparatus according to the first embodiment of the present invention.

[0016]FIG. 4 is a perspective view of a prism base of the image display
apparatus according to the first embodiment of the present invention.

[0017]FIG. 5 is an exploded perspective view of an assembly in the image
display apparatus of the first embodiment according to the present
invention.

[0018]FIG. 6 is a perspective view of the assembly in the image display
apparatus according to the first embodiment of the present invention,
from which a quarter-wave plate holder is removed.

[0019]FIG. 7 is a perspective view of the assembly in the image display
apparatus according to the first embodiment of the present invention,
from which a mask base and a panel mask are removed.

[0020]FIG. 8 is a perspective view of an assembled state of the assembly
in the image display apparatus according to the first embodiment of the
present invention.

[0021]FIG. 9 is perspective view of an assembly in an image display
apparatus according to a second embodiment of the present invention,
which includes a quarter-wave plate and a liquid crystal display device.

DESCRIPTION OF THE EMBODIMENTS

[0022]FIG. 1 is plan view, and FIG. 2 is a side view of an optical system
of a projector-type image display apparatus according to a first
embodiment of the present invention. An arc tube 1 and a reflector 2
constitute a light source lamp 3. In front of the light source lamp 3, an
explosion-proof glass 4, a first cylinder array 5a, a UV filter 6, a
second cylinder array 5b, a polarization conversion element 7, a front
compressor 8, and a total reflection mirror 9 are arranged. In the
reflection direction of the total reflection mirror 9, a third cylinder
array 5c, a color filter 10, a fourth cylinder array 5d, a condenser lens
11, and a rear compressor 12 are arranged. Such elements from the light
source lamp 3 to the rear compressor 12 constitute an illumination
optical system 13.

[0023]Light exit from the illumination optical system 13 enters a color
separation and combination optical system 14, which corresponds to a
color separation unit and a color combination unit of the present
invention. In the color separation and combination optical system 14, an
incident side polarizer 16 for green light, a first polarizing beam
splitter 17, a quarter-wave plate 18G, and a reflective liquid crystal
display device 19G for green light are disposed in the transmission
direction of a dichroic mirror 15. In the reflection direction of the
first polarizing beam splitter 17, a dichroic prism 21, to which an exit
side polarizer 20G for green light is attached, is disposed. In the
reflection direction of the dichroic mirror 15, a trimming filter 22, an
incident side polarizer 16a, a color selective retardation plate 23, a
second polarizing beam splitter 24, a quarter-wave plate 18B, and a
reflective liquid crystal display device 19B are arranged.

[0024]In the direction in which the second polarizing beam splitter 24
reflects the light beams entering from the dichroic mirror 15, a
quarter-wave plate 18R and a reflective liquid crystal display device 19R
for red light are disposed. In the exit side of the second polarizing
beam splitter 24, the dichroic prism 21, to which an exit side polarizer
20B for blue light is attached, is disposed. Light that exits the
dichroic prism 21 reaches a projection lens barrel 25.

[0025]The arc tube 1 emits continuous spectrum white light. The reflector
2 reflects the light from the arc tube 1 in a specified direction. The
first cylinder array 5a has a refractive power in the direction
perpendicular to the path of light from the light source lamp 3 and
perpendicular to the plane of the page. The second cylinder array 5b has
a lens array, each lens of which corresponds to a lens in the first
cylinder array 5a. The polarization conversion element 7 converts
unpolarized light into specified polarized light.

[0026]The front compressor 8 is constituted by a cylindrical lens having a
refractive power in a direction perpendicular to the light path. The
total reflection mirror 9 changes the direction of an optical axis by an
angle of 88 degrees. The third cylinder array 5c has a refractive power
in a direction perpendicular to the light path from the light source lamp
3, i.e., in a direction perpendicular to the plane of the page. The
fourth cylinder array 5d has a lens array, each lens of which corresponds
to a lens of the third cylinder array 5c. The color filter 10 returns
colored light in a specified wavelength band to the light source lamp 3
so as to adjust color coordinates to specified values. The rear
compressor 12 is constituted by a cylindrical lens having a refractive
power in a direction perpendicular to the light path.

[0028]An optical image is formed in accordance with an image signal. The
reflective liquid crystal display devices 19R, 19G, and 19B modulate
light incident on the devices and reflect the light via the respective
quarter-wave plates 18R, 18G, and 18B. The trimming filter 22 returns
orange light to the light source lamp 3 so as to increase the color
purity of red light R. The incident side polarizer 16a for red light R
and blue light B, in which a polarizing element is attached to a
transparent substrate, transmits p-polarized light only. The color
selective retardation plate 23 changes the polarization direction of red
light R by an angle of 90 degrees while leaving the polarization
direction of blue light B unchanged. The second polarizing beam splitter
24 has a polarization separation surface which transmits p-polarized
light and reflects s-polarized light.

[0030]Note that the terms "p-polarized" and "s-polarized" are used as
follows. The polarization conversion element 7 converts p-polarized light
to s-polarized light from the viewpoint of the polarization conversion
element 7. On the other hand, light that enters the dichroic mirror 15 is
said to be p-polarized from the viewpoint of the polarizing beam
splitters 17 and 24. So, although light exiting from the polarization
conversion element 7 is s-polarized light from the viewpoint of the
polarization conversion element 7, this same s-polarized light that
enters the dichroic mirror 15 is said to be p-polarized light from the
viewpoint of the polarizing beam splitters 17 and 24.

[0031]Light emitted from the arc tube 1 is collected in a specified
direction by the reflector 2 having a parabolic shape. Light emitted from
the focus of a paraboloid becomes light beams parallel to the axis of
symmetry of the paraboloid. However, since the arc tube 1 has a finite
size is thus not an ideal light source, the collected light beams include
many light components that are not parallel to the axis of symmetry of
the paraboloid.

[0032]The light beams enter the first cylinder array 5a. In the first
cylinder array 5a, the light beams are divided and collected into light
beams each of which are strip-shaped extending in a direction
perpendicular to the light path and corresponding to a cylinder array
lens. Moreover, the light beams pass through the UV filter 6 and the
second cylinder array 5b so that the light beams each of which are
strip-shaped in a direction perpendicular to the light path are formed in
the vicinity of the polarization conversion element 7.

[0033]The polarization conversion element 7 includes polarization
separation surfaces, reflection surfaces, and half-wave plates. Each of
the light beams is incident on a corresponding polarization separation
surface and divided into a p-polarized component that is transmitted and
an s-polarized component that is reflected. The reflected s-polarized
component reflects off the reflection surface and exits in the same
direction as the p-polarized component. The transmitted p-polarized
component passes through the half-wave plate so as to be converted into a
component having the same polarization as the s-polarized component, and
both components exit the polarization conversion element 7 as light
having a uniform polarization direction. The polarization-converted light
beams, each of which is strip-shaped in a direction perpendicular to the
light path, exit the polarization conversion element 7. Then, the light
beams pass through the front compressor 8, reflect off the total
reflection mirror 9 at an angle of 88 degrees, and enter the third
cylinder array 5c. The light beams which entered the third cylinder array
5c are divided into light beams, each of which is strip-shaped in the
horizontal direction in accordance with a cylinder array lens, and
collected. The light beams pass through the fourth cylinder array 5d,
where the light beams are made into light beams each having a band-shape
in the horizontal direction, pass through the condenser lens 11, and
reach the rear compressor 12.

[0034]Due to optical functions of the front compressor 8, the condenser
lens 11, and the rear compressor 12, the light beams provide a
rectangular and evenly luminous area in which rectangular images are
superposed on top of one another. Reflective liquid crystal display
devices 19R, 19G, and 19B, which are described below, are disposed in the
luminous area. Next, the light that was converted to s-polarized light by
the polarization conversion element 7 enters the dichroic mirror 15.

[0035]Green light G, which passed through the dichroic mirror 15, enters
the incident side polarizer 16 for green light. The green light G
continues to be s-polarized light after being separated by the dichroic
mirror 15, when the polarization conversion element 7 is of a
p-polarizing type. The green light G exits the incident side polarizer 16
for green light, enters the polarizing beam splitter 17 as p-polarized
light, passes through the polarization separation surface, and reaches
the reflective liquid crystal display device 19G for green light. The
green light G is image-modulated and reflected by the reflective liquid
crystal display device 19G for green light. The p-polarized component of
the image-modulated and reflected green light G is returned through the
polarization separation surface of the first polarizing beam splitter 17
toward the light source lamp 3 so as to be removed from the projection
light. On the other hand, the s-polarized component of the
image-modulated green light G is reflected by the polarization separation
surface of the first polarizing beam splitter 17 and travels toward the
dichroic prism 21 so as to be projected.

[0036]In a state in which all the polarized components are converted to be
p-polarized, i.e., in a state that black is displayed, the slow axis of
the quarter-wave plate 18G, which is placed between the first polarizing
beam splitter 17 and the reflective liquid crystal display device 19G for
green light, is aligned in a specified direction. This alignment serves
to reduce the influence of polarization fluctuations generated in the
first polarizing beam splitter 17 and the reflective liquid crystal
display device 19G for green light. The green light G, which exited the
first polarizing beam splitter 17, enters the dichroic prism 21 as
s-polarized light. The green light G reflects off the dichroic coating of
the dichroic prism 21 and reaches the projection lens barrel 25.

[0037]Red light R and blue light B reflected by the dichroic mirror 15
enter the incident side polarizer 16a. The red light R and the blue light
B continue to be p-polarized light after being separated by the dichroic
mirror 15. The trimming filter 22 filters out orange light from the red
light R and the blue light B. Then, the red light R and the blue light B
exit the incident side polarizer 16a and enter the color selective
retardation plate 23. The color selective retardation plate 23 rotates
the polarization direction by an angle of 90 degrees only for red light
R. Thus, the red light R as s-polarized light and the blue light B as
p-polarized light enter the second polarizing beam splitter 24. The red
light R, which entered the second polarizing beam splitter 24 as
s-polarized light, is reflected by the polarization separation surface of
the second polarizing beam splitter 24 and reaches the reflective liquid
crystal display device 19R for red light. The blue light B, which entered
the second polarizing beam splitter 24 as p-polarized light, passes
through the polarization separation surface of the second polarizing beam
splitter 24 and reaches the reflective liquid crystal display device 19B
for blue light.

[0038]The red light R enters the reflective liquid crystal display device
19R, whereby the red light R is image-modulated and reflected. The
s-polarized component of the image-modulated red light R is reflected
again by the polarization separation surface of the second polarizing
beam splitter 24, returned toward the light source, and removed from the
projection light. On the other hand, the p-polarized component of the
image-modulated red light R passes through the polarization separation
surface of the second polarizing beam splitter 24 and travels toward the
dichroic prism 21 so as to be projected.

[0039]The blue light B enters the reflective liquid crystal display device
19B, whereby the blue light B is image-modulated and reflected. The
p-polarized component of the image-modulated blue light B again passes
through the polarization separation surface of the second polarizing beam
splitter 24, returned toward the light source lamp 3, and removed from
the projection light. On the other hand, the s-polarized component of the
image-modulated blue light B is reflected by the polarization separation
surface of the second polarizing beam splitter 24 and travels toward the
dichroic prism 21 so as to be projected.

[0040]By aligning the slow axes of the quarter-wave plates 18R and 18B,
which are placed between the second polarizing beam splitter 24 and the
reflective liquid crystal display devices 19R and 19B, black displays of
the red light R and the blue light B can be adjusted as in the case of
the green light G.

[0041]The blue light B, which is included in the red light R and blue
light B that have been combined into a single light beam and exit from
the second polarizing beam splitter 24, is analyzed by the exit side
polarizer 20B for blue light and enters the dichroic prism 21. The red
light R, while maintaining the p-polarized state, passes through the exit
side polarizer 20B for blue light and enters the dichroic prism 21.

[0042]Unnecessary components were generated while the blue light B passed
through the second polarizing beam splitter 24, reflective liquid crystal
display device 19B for blue light, and the quarter-wave plate 18B. When
the blue light B is analyzed by the exit side polarizer 20B for blue
light, such unnecessary components are removed.

[0043]The red light R and the blue light B enter the dichroic prism 21 and
pass through the dichroic coating of the dichroic prism 21. Then, the red
light R and the blue light B are combined with the green light G
reflecting off the dichroic coating and reach the projection lens barrel
25.

[0044]The red light R, green light G, and blue light B, which have been
combined, are magnified and projected through the projection lens barrel
25 onto a projection surface, such as a screen.

[0045]The above-described optical path corresponds to a case when the
reflective liquid crystal display devices are in a white display state.
In the following, an optical path for a case when the reflective liquid
crystal display devices are in a black display state is described. The
p-polarized component of the green light G, which passed through the
dichroic mirror 15, enters the incident side polarizer 16 for green light
and the first polarizing beam splitter 17, passes through the
polarization separation surface of the first polarizing beam splitter 17,
and reaches the reflective liquid crystal display device 19G for green
light. However, since the reflective liquid crystal display device 19G
for green light is in a black display state, the green light G is
reflected without being image-modulated. For this reason, the green light
G continues to be p-polarized after being reflected by the reflective
liquid crystal display device 19G for green light. Therefore, the green
light G again passes through the polarization separation surface of the
first polarizing beam splitter 17, passes through the incident side
polarizer 16 for green light, and is returned toward the light source
lamp 3 so as to be removed from the projection light.

[0046]The p-polarized components of the red light R and the blue light B,
which were reflected by the dichroic mirror 15, enter the incident side
polarizer 16a, exit the incident side polarizer 16a, and enter the color
selective retardation plate 23. The color selective retardation plate 23
has a function to rotate the polarization direction by an angle of 90
degrees for only the red light R. Due to this function, the red light R
enters the second polarizing beam splitter 24 as s-polarized light, and
the blue light B enters the second polarizing beam splitter 24 as
p-polarized light. The red light R, which entered the second polarizing
beam splitter 24 as s-polarized light, is reflected by the polarization
separation surface of the second polarizing beam splitter 24 and reaches
the reflective liquid crystal display device 19R for red light.

[0047]The blue light B, which entered the second polarizing beam splitter
24 as p-polarized light, passes through the polarization separation
surface of the second polarizing beam splitter 24 and reaches the
reflective liquid crystal display device 19B for blue light. Since the
reflective liquid crystal display device 19R for red light is in a black
display state, the red light R that entered the reflective liquid crystal
display device 19R for red light is reflected without being
image-modulated. For this reason, the red light R continues to be
s-polarized after being reflected by the reflective liquid crystal
display device 19R for red light. Therefore, the red light R is reflected
again by the polarization separation surface of the first polarizing beam
splitter 17, passes through the incident side polarizer 16a, and is
returned toward the light source so as to be removed from the projection
light and display black.

[0048]On the other hand, the blue light B that entered the reflective
liquid crystal display device 19B for blue light is reflected without
being image-modulated, because the reflective liquid crystal display
device 19B for blue light is in a black display state. For this reason,
the blue light B continues to be p-polarized after being reflected by the
reflective liquid crystal display device 19B for blue light. Therefore,
the blue light again passes through the first polarizing beam splitter
17. Then, the blue light is converted to p-polarized light by the color
selective retardation plate 23. After passing through the incident side
polarizer 16a, the blue light is returned toward the light source lamp 3
so as to be removed from the projection light.

[0049]FIG. 3 is an exploded perspective view of the projection-type image
display apparatus. Light from the light source lamp 3 travels through the
illumination optical system 13. The color separation and combination
optical system 14 includes the liquid crystal devices for red light R,
green light G, and blue light B, on which the light from the illumination
optical system 13 is incident. The light from the color separation and
combination optical system 14 enters the projection lens barrel 25,
whereby the image is projected onto a screen (not shown). The projection
lens barrel 25 contains a projection lens optical system.

[0050]An optical box 30 contains the light source lamp 3, the illumination
optical system 13, and the color separation and combination optical
system 14. The projection lens barrel 25 is fixed to the optical box 30.
The optical box 30, containing the illumination optical system 13 and the
color separation and combination optical system 14, is covered with a lid
31. A power supply filter 32 and a ballast power supply 33, which are
connected to a power supply 34, supply the light source lamp 3 with
electric current for lighting the light source lamp 3. A circuit board 35
issues commands for driving the liquid crystal devices and lighting the
light source lamp 3 using electric power from the power supply 34.
Optical system cooling fans 36a and 36b draw in air through an air intake
port 38a of an exterior housing 37 so as to cool optical elements, such
as the liquid crystal devices, in the color separation and combination
optical system 14. Air drawn in by the optical system cooling fans 36a
and 36b is delivered through a duct 39 to the optical elements, such the
liquid crystal devices, in the color separation and combination optical
system 14.

[0051]A lamp cooling fan 40 blows air to the light source lamp 3 so as to
cool the light source lamp 3. A lamp duct 41, while holding the lamp
cooling fan 40, serves to deliver the cooling air to the light source
lamp 3. A lamp duct 42, which clamps the lamp cooling fan 40, constitutes
a duct together with the lamp duct 41. A power supply cooling fan 43
draws in air through an air intake port 38b of the exterior housing 37 so
that air circulates inside the power supply 34 and the ballast power
supply 33, thereby cooling the power supply 34 and the ballast power
supply 33 simultaneously. An exhaust fan 44 discharges air that was blown
by the lamp cooling fan 40 and heated by the light source lamp 3 from the
exterior housing 37.

[0052]Lamp exhaust louvers 45 and 46 have a shielding function whereby
light from the light source lamp 3 is not leaked to the outside of the
apparatus. The exterior housing 37 contains the optical box 30 and other
elements. An exterior housing lid 47 covers the exterior housing 37
containing the optical box 30 and other elements. The exterior housing 37
is closed with side panels 48 and 49. The side panel 49 has an exhaust
port 49a. Connectors for receiving various signals are mounted on an
interface substrate 50. An interface reinforcing plate 51 is attached to
the inner surface of the side panel 48.

[0053]A lamp exhaust box 52 serves to transfer heat from the light source
lamp 3 to the exhaust fan 44 and prevent exhaust air from spreading
inside of the exterior housing 37. The lamp exhaust box 52 holds the lamp
exhaust louvers 45 and 46. The lamp lid 53 is screwed onto the bottom
surface of the exterior housing 37 in a removable manner. An adjustable
foot 54 is attached to the exterior housing 37. The height of a foot
portion 54a of the adjustable foot 54 can be adjusted so as to adjust the
tilt angle of the main body of the apparatus.

[0054]An RGB plate 55 holds a filter (not shown) attached to the outside
of the air intake port 38a of the exterior housing 37. An RGB board 56 is
disposed in the color separation and combination optical system 14. The
RGB board 56 is connected to a flexible printed circuit (FPC) extending
from the reflective liquid crystal display devices and is connected to
the circuit board 35. An RGB board cover 57 is provided so as to prevent
electric noise from entering the RGB board 56.

[0055]A prism base 58 holds the color separation and combination optical
system 14. A box side cover 59 has a duct-shaped portion that guides
cooling air from the optical system cooling fans 36a and 36b so as to
cool the optical elements of the color separation and combination optical
system 14 and the reflective liquid crystal display devices. An RGB duct
60 forms a duct together with the box side cover 59.

[0057]FIG. 5 is an exploded perspective view of an assembly including a
quarter-wave plate 18, a light-shielding member including a mask base 73
and a panel mask 74, and the reflective liquid crystal display device 19.
The structure is the same for red, green, and blue light. The reflective
liquid crystal display device 19 is fixed to a heatsink 71. On the
heatsink 71, the mask base 73 serving as a base member, the panel mask 74
serving as a mask member, a rubber shield 75, and a quarter-wave plate
holder 72, to which the quarter-wave plate 18 is attached, are stacked
upon each other. The mask base 73 and the panel mask 74 constitute the
light-shielding member. The panel mask 74 shields light so that
illuminating light is not incident on the area outside the effective
display area of the reflective liquid crystal display device 19.

[0058]FIG. 6 is a perspective view of the assembly shown in FIG. 5 in a
state in which the elements from the heatsink 71 to the rubber shield 75
are stacked upon each other. The reflective liquid crystal display device
19 is fixed to the heatsink 71. Thermally-conductive silicone is applied
between the heatsink 71 and the reflective liquid crystal display device
19 so that heat of the reflective liquid crystal display device 19, which
is generated by absorption of light, is effectively dissipated to the
heatsink 71.

[0059]Lugs 73a on the mask base 73 engage with hooks 74a on the panel mask
74 so as to be fixed. The lugs 73a and the hooks 74a are disposed at four
positions along the longitudinal sides. A protrusion at the center of
each of the lugs 73a is elastically deformed and put into a recess in the
corresponding one of the hooks 74a, thereby serving as a stopper in the
direction of the optical axis. Since the panel mask 74 is fixed to the
mask base 73 using elastic deformation, the panel mask 74 can be easily
detached from the mask base 73.

[0060]In this embodiment, the mask base 73 may be made by aluminum
die-casting or plastic molding. Alternatively, the mask base 73 may be
made of sheet metal or the like.

[0061]The mask base 73 is fixed to the heatsink 71 after the position of
the mask base 73 is adjusted on the heatsink 71. In order to fix the mask
base 73 to the heatsink 71, a UV adhesive is applied to four bonding
portions 73b on the mask base 73, the positions of the bonding portions
are adjusted, and then the UV adhesive is cured by irradiating the
adhesive with UV rays.

[0062]The rubber shield 75, which serves as an elastic member, is mounted
on the panel mask 74. The rubber shield 75 has a flange portion 75a. A
side edge 74b of the panel mask 74 is inserted into a narrow recess in
the back side of a side edge 75b of the rubber shield 75. Similarly, a
side edge 74c is inserted into a recess in the back side of a side edge
75c. With such insertion on the opposite sides, the rubber shield 75 is
held on the panel mask 74.

[0063]Since the rubber shield 75 is made of an elastic material such as
rubber, the rubber shield 75 is not easily fixed to the panel mask 74
with screws or adhesives. Nevertheless, it is preferable that the
component can be removed so that any accumulated dust can be easily
cleaned therefrom. The above-described structure may be adopted to
promote easy cleaning.

[0064]The flange portion 75a of the rubber shield 75 surrounds a space
formed by the reflective liquid crystal display device 19 and the
quarter-wave plate holder 72, which is an optical element disposed at a
side of the light source of the reflective liquid crystal display device
19, and seals the space so as to prevent dust from depositing on the
light entry/exit surface of the reflective liquid crystal display device
19.

[0065]The quarter-wave plate 18 is attached to a frame-shaped portion of
the quarter-wave plate holder 72 and fixed by an adhesive. In order to
achieve a high contrast, the quarter-wave plate 18 is disposed with high
accuracy and held by the quarter-wave plate holder 72 without looseness.
Although an adhesive is used in this embodiment, a plate spring or a
screw may be used for fixing.

[0066]Since an image on the liquid crystal display device 19 is projected
onto the screen, dust on a surface of the reflective liquid crystal
display device is particularly conspicuous. In this embodiment, dust on
the surface of the liquid crystal display device 19 is the most
conspicuous, and dust on the surface of the quarter-wave plate 18
adjacent to the liquid crystal display device 19 is the second most
conspicuous. On the other hand, dust on the surface of the quarter-wave
plate 18 adjacent to the projection side is less conspicuous, because the
light is less focused at the surface.

[0067]As described above, dust deposited on the surface of the liquid
crystal display device 19, which tends to be conspicuous, is removed.
High-pressure air may be blown so as to remove the dust. However,
extremely small pieces of dust might not be removed by blowing
high-pressure air. For this reason, a method of wiping the surface with
silbon paper impregnated with alcohol solvent may be used. Because
alcohol solvent often leaves wiping marks, the surface of the liquid
crystal display device 19 is preferably wiped with one stroke. At that
time, the surface of the liquid crystal display device 19 should be fully
exposed.

[0068]FIG. 7 shows a state when the panel mask 74 is removed from the mask
base 73 to allow cleaning of the surface of the liquid crystal display
device 19. Because the mask base 73 has an opening larger than the light
entry/exit surface of the liquid crystal display device 19, the surface
of the liquid crystal display device 19 is fully exposed in the state
shown in FIG. 7. Therefore, the surface can be wiped with one stroke with
silbon paper and alcohol solvent. After the dust on the surface of the
liquid crystal display device is removed, the panel mask 74 is mounted on
the mask base 73 as shown in FIG. 6. Then, the rubber shield 75 and the
quarter-wave plate holder 72 are stacked upon each other, thereby
providing an assembly shown in FIG. 8.

[0069]With the structure in which the mask base 73 and the panel mask 74
are detachable, dust deposited on the surface of the liquid crystal
display device 19 can be easily removed. Moreover, the position of the
mask base 73 is adjusted relative to the effective display area of the
liquid crystal display device 19 and the mask base 73 is fixed to the
liquid crystal display device 19. Therefore, even when the mask base 73
is detached from the panel mask 74 in order to remove dust deposited on
the surface of the liquid crystal display device, the position of the
mask base 73 is hardly displaced relative to the effective display area
of the liquid crystal display device.

[0070]In the assembly shown in FIG. 8, the rubber shield 75 blocks dust,
thereby providing an excellent dustproofness. In a prism unit shown in
FIG. 4, the liquid crystal display device and the light-shielding frame
of the panel mask 74 are disposed with high accuracy. Therefore, color
smudge in a periphery of the projection image due to a displacement of
relative position of the light-shielding frame is reduced, and an
excellent image without dust-related deterioration is provided.

[0071]FIG. 9 is a perspective view of a second embodiment. In the first
embodiment, the mask base 73 is fixed to the heatsink 71 with a UV
adhesive. In the second embodiment, the mask base 73 is fixed to the
heatsink 71 is fixed with screws 76.

[0072]When the liquid crystal display device 19 is large, the mask base 73
needs to be heavy. Therefore, the position of the mask base 73 may be
displaced due to vibration or a drop impact. In such a case, the mask
base 73 can be solidly fixed to the heatsink 71 by applying and curing a
small amount of UV adhesive for temporary securement and then securing
with the screws 76. Washers are preferably used so as to prevent
displacement of the panel mask 74 caused when securing the mask base 73
with the screws 76.

[0073]While the present invention has been described with reference to
exemplary embodiments, it is to be understood that the invention is not
limited to the disclosed exemplary embodiments. The scope of the
following claims is to be accorded the broadest interpretation so as to
encompass all modifications and equivalent structures and functions.

[0074]This application claims the benefit of Japanese Patent Application
No. 2008-030006 filed Feb. 12, 2008, which is hereby incorporated by
reference herein in its entirety.